Acrylonitrile polymerization using sodium sulfoxylate formaldehyde

ABSTRACT

Polymerization of acrylonitrile in aqueous system using watersoluble redox catalyst system comprising oxidizing catalyst and reducing bisulfite activator is improved by (a) conducting the polymerization in the presence of 0.1 to 0.5 parts of sodium sulfoxylate formaldehyde per 100 parts of monomer to inhibit cyanide by-product formation and/or (b) the use of sodium sulfoxylate formaldehyde to shortstop the polymerization reaction while minimizing the formation of undesirable water-soluble acrylonitrile-bisulfite addition product.

United States Patent Chan Oct. 28, 1975 ACRYLONITRILE POLYMERIZATIONUSING SODIUM SULFOXYLATE [56] References Cited FORMALDEHYDE UNITEDSTATES PATENTS lnvemorl David Yee Hing Chan Pensacola 2,546.238 3/1951Richards 260/887 R 2,560,694 7/1951 Howard 260/88.7 D Assigneez AmericanCyanamid p y, 3,058.937 10/1962 Furness 260/855 D Stamford, Conn. vPrimary Examiner-Joseph L. Schofer [22] Flled: 7, 1974 AssistantE.\'aminerl-lerbert J. Lilling [21] Appl No; 440,419 Attorney, Agent, orFirnlPhilip Mintz Related U.S. Application Data ABSTRACT [62] Divisionof Ser. No. 297,551, Oct. 13, I972, Pat. No.

3,321,171 Polymerization of acrylonitrile in aqueous system usingwater-soluble redox catalyst system comprising [52] US, Cl, 260/855 R;260/63 N; 260/793 M; oxidizing catalyst and reducing bisulfite activatoris 2 55 2 0 355 2 0 355 improved by (a) conducting the polymerization inthe 260/855 HC; 260/855 1,;260/5355 XA; presence of 0.1 to 0.5 parts ofsodium sulfoxylate 260/88.7 R; 260/88] B; 260/887 D; formaldehyde perparts of monomer to inhibit cy- 260/38 7 E anide by-product formationand/or (b) the use of so- [51] Int. Cl. C08F 220/42, COSF 220/70, diumsulfoxylate formaldehyde to shortstop the poly- C08 /42 @031: 120/70merization reaction while minimizing the formation of 5 Field f Search 20 55 R, 55 S, 55 D, undesirable water-soluble acrylonitrile-bisulfiteaddi 260/855 L, 88.7 R, 88.7 B, 88.7 D, 88.7 E, 85.5 ES, 85.5 HC:85.5 XA

tion product.

6 Claims, No Drawings ACRYLONITRHLE POLYMERTZATTON USING SOlDllUh/lSULFOXYLATE FORMALDEHYDE This is a division of application Ser. No.297,551, filed Oct. 13, 1972, now U.S. Pat. No. 3,821,177.

This invention relates to polymerization of acryloni trile and recoveryof unreacted monomer from such polymerization while minimizing theformation of undesirable by-products.

It is well-known to polymerize acrylonitrile, alone or with one or moreethylenically unsaturated monomers copolymerizable therewith, in aqueoussystems using water-soluble redox catalyst systems comprising anoxidizing catalyst and a reducing bisulfite activator. In

such process, it is common practice to stop the poly merization reactionprior to complete polymerization of all monomers and, thereafter, torecover the unreacted monomers from the slurry which contains water,polymer, unreacted monomer, catalyst residues, etc. for reuse. Usuallysuch monomer recovery involves evaporation of unreacted monomers eitherfrom the polymerization slurry directly or from the filtrate afterremoval of the insoluble polymer by filtering the slurry.

In accordance with the present invention, this polymerization andmonomer recovery process is improved by (a) the use of 0.1 to 0.5 partsof sodium sulfoxylate formaldehyde per 100 parts of monomer fed to thepolymerization reactor along with monomer, redox cata lyst systemcomprising oxidizer catalyst and bisulfite activator, water, etc. toinhibit cyanide by'product formation or (b) the addition of about 1%, onweight of unreacted monomer, of sodium sulfoxylate formaldehyde to thepolymerization slurry to short-stop the polymerization reaction whileminimizing the formation of undesirable water-solubleacrylonitrile-bisulfite addition product, or both (a) and (b) incombination.

Acrylonitrile polymers find wide use in the production of syntheticfibers. Such polymers may be made by polymerizing acrylonitrile alone,or in combination with one or more ethylenically undaturated monomerscopolymerizable therewith, such as those mentioned, for instance, in US.Pat. Nos. 3,104,938 and 3,040,008 and the various other patentsmentioned therein. Such monomers include, but are not limited to, vinylacetate, methyl acrylate, methyl methacrylate, vinylpyridines, styrene,styrene sulfonic acid, vinyl chloride, vinylidene chloride, acrylicacid, acrylamide, methyl vinyl ketone, etc. Such acrylonitrile polymersnormally contain a major proportion (more than 50%) acrylonitrile, andpreferably, more than 70% acrylonitrile with the remainder being suchother ethylenically unsaturated comonomers. The present invention isuseful in connection with producing such acrylonitrile polymers.

Acrylonitrile polymers can be prepared by the use of freeradicalpolymerization initiators of several types. Redox catalyst systems,comprising an oxidizing catalyst and a reducing activator, are currentlyin wide commercial use. Especially preferred commercially are thoseredox catalyst systems which utilize a reducing bisulfite activator.lllustrative of these bisulfite activators are materials which producebisulfite ions when dissolved in an aqueous acidic oxidizing medium,such as alkali metal sulfites, bisulfites, metabisulfites,hydrosulfites, and thiosulfates, and sulfur dioxide. illustrative of theconventional oxidizing catalysts used with such reducing bisulfiteactivators are alkali metal chlorates 2 and persulfates. Such redoxcatalyst systems are water soluble and are used to polymerizeacrylonitrile in an aqueous polymerization medium which is maintained ata low pH, usually by the addition of acid, usually a strong inorganicacid such as nitric or sulfuric acid. It is in connection with use ofthese well-known redox cata lyst systems for polymerizing acrylonitrilethat the present invention is concerned.

When the polymerization reaction has proceeded to the point where thedesired proportion of monomer has been converted to polymer, usuallyabout 60 to percent conversion, the reaction is stopped by the additionof a shortstopping agent. Thereafter, the slurry, which contains water,polymer unreacted monomer, catalyst residues, is processed to recoverunreacted monomer for reuse and polymer for further treatments leadingto final products. Usually such processing involves evaporation ofunreacted monomers either from the polymer slurry directly or from thefiltrate after filtering the slurry to remove insoluble polymer. Thereasons why stopping the polymerizationrcaction while a substantialproportion of the monomers are unreacted, desirable characteristicssought in shortstopping agents, and previously proposed shortstoppingagents may be found in the prior art, representative of which areRichards US. Pat. No. 2,546,238; Howland et a1. U.S. Pat. No. 2,556,651;Hieserman et al. US. Pat. No. 3,084,143; Himes et al. US. Pat. No.3,091,602; Thompson et al. US. Pat. No. 3,153,024; Nakajima et a1. U.S.Pat. No. 3,192,189; Sampson et al. US. Pat. No. 3,308,109; and Cheape etal. US. Pat. No. 3,454,542.

As is generally true with complex organic chemical processes, theabove-described prior art processes for polymerizing acrylonitrile andrecovering unreacted monomer produce unwanted by-products due tounintended reactions among the many reactants concurrently present inthe same system. The production of such by-products is undesirable formany reasons, among which are wastage of raw materials and creation ofdeleterious substances which must be recovered from process streams atconsiderable expense or discarded as contaminants in waste water withconcommitant polution of natural waterways. The present invention isparticularly concerned with minimizing the production of two of theby-products normally produced by this process.

During the polymerization of acrylonitrile in an aqueous system by usinga water-soluble redox catalyst system comprising an oxidizing catalystand a reducing bisulfite activator, hydrogen cyanide is formed as one ofthe by-products. It is soluble in water and in acrylonitrile monomerand, during the postpolymerization processing of the polymerizationslurry to recover unreacted monomers for reuse, it distills over withthe acrylonitrile and is recycled into the polymerization reactor. Theconcentration of hydrogen cyanide in the system resulting from thisrecycling gradually increases and eventually this by-product finds itsway into various effluent streams where, if particular care is not takento remove it, it finds its way into natural waterways. ln accordancewith the present invention, the amount of hydrogen cyanide formed inthis process is sharply reduced by the use of 0.1 to 0.5 parts of sodiumsulfoxylate formaldehyde per parts of monomer fed to the polymerizationreactor. While the method of adding the sodium sulfoxylate formaldehydeto the polymerization reactor is not critical, it is preferablyintroduced 3 as an aqueous solution to ensure good mixing. Use of lessthan 0.1 part sodium sulfoxylate formaldehyde per 100 parts of monomerdoes not sufficiently inhibit formation of hydrogen cyanide while use ofmore than 0.5 part seriously reduces the proportion of the monomerconverted to polymer to an unacceptable level.

When the polymerization reaction has reached the desired conversionlevel, the reaction is stopped by addition of a shortstopping agent.Most often. the shortstopping agent serves, inter alia, to raise the pHof the polymerization slurry to above about 5. It has been observed thatat these elevated pH values, an acrylonitrile-bisulfite addition productis formed from unreacted acrylonitrile monomer and unconsumed bisulfiteactivator of the redox catalyst system. Although at low pH values.formation of the acrylonitrile-bisulfite addition product can beminimized, it is difficult to find shortstopping agents which areeffective at such low pH values. Provided the polymerization slurry isdistilled to remove unreacted monomers immediately after shortstopping,Cheape et al. US. Pat. No. 3,454,542 suggests use of oxalic acid as theshortstopping agent. However, there may be occasions where it isdesirable to use another, different shortstopping agent. Useable agentscapable of shortstopping this reaction without increasing the pH toabove 5 are difficult to find. In accordance with the present invention.the polymerization reaction is shortstopped by adding sufficient sodiumsulfoxylate formaldehyde to the polymerization slurry to inhibit furtherpolymerization, preferably about 1% on weight of unreacted monomeralthough more may be used. This shortstopping agent is effective at lowpH values (it does not materially change the pH of the polymerizationslurry) which inhibits the formation of acrylonitrile-bisulfite additionproduct for an extended period. While the method of adding the sodiumsulfoxylate formaldehyde to the polymerization slurry is not critical,it is preferably added as an aqueous solution to ensure good mixing.

Although it is preferable to utilize both additions of sodiumsulfoxylate formaldehyde in combination, either can be used alone toreduce unwanted by-product formation as described above.

The following examples will serve to illustrate preferred embodiments ofthis invention. All proportions are by weight unless otherwiseindicated.

EXAMPLE 1 A water-jacketed reactor having a volume of 6.78 liters isprovided with a propeller-type stirrer driven by a motor which rotatesat about 900 RPM. The reactor is provided with a four-feed deliverysystem and, at its top, an overflow tube. To start-up for continuouspolymerization using this reactor, it was filled with water containinggrams of sodium metabisulfite adjusted to a pH of about 2.7 with nitricacid and warmed to 55C. During continuous polymerization, four feedliquids are metered into the reactor continuously, the reactiontemperature is maintained at 55C., and polymerization slurrycontinuously overflows through the overflow tube.

To serve as a basis of comparison, a control run without use of sodiumsulfoxylate formaldehyde was performed using the following four feedcompositions and feed rates:

Feed I 90.6% acrylonitrile and 9.4% methyl methacrylate monomerscontaining 5 ppm hydrogen cyanide at 2,41 l cc./hr.

Feed 11 Aqueous solution of 10% sodium metabisulfite reducing activatorat 591 cc./hr.

Feed 111 Aqueous solution of 0.9% sodium chlorate oxidizing catalyst at745 cc./hr.

Feed IV Deionized water at 3,033 cc./hr.

During the course of the polymerization reaction, the pH remains atabout 2.52.6. After equilibrium conditions had been attained (afterseveral hours of operation), samples were taken of the reactor overflowslurry for analysis. The slurry contained about 25.7% polymer solids,the conversion of monomer to polymer was about 85.6%, the polymercomposition was about 89.3% acrylonitrile and 10.7% methyl methacrylate,the polymer had an intrinsic viscosity of 1.48 and a weight averagemolecular weight of about 1 14,000, and the polymer was found to containabout 0.133% sulfur. At equilibrium, the polymerization slurry was foundto contain about 12-13 ppm hydrogen cyanide.

EXAMPLE 2 To illustrate the present invention, Example l was repeatedexcept that Feed 111 also contained 0.75% sodium sulfoxylateformaldehyde and there were minor changes in feed rates (viz., Feed I2,411 cc./hr.; Feed 11 709 cc./hr.; Feed III 894 cc./hr.; and Feed IV2,766 cc./hr.). Thus, in this example, approximately 0.35 part of sodiumsulfoxylate formaldehyde was fed to the reactor for each parts ofmonomer.

During the course of the polymerization, the pH remains at about 2.62.7.After equilibrium conditions had been attained, analysis of slurrysamples showed that the slurry contained about 25.3% polymer solids, theconversion of monomer to polymer was about 84.3%, the polymercomposition was about 89.3% acrylonitrile and 10.7% methyl methacrylate,the polymer had an intrinsic viscosity of 1.46 and a weight averagemolecular weight of about 110,000, and the polymer was found to contain0.131% sulfur. At equilib rium, the polymerization slurry was found tocontain about 2.8-4.2 ppm hydrogen cyanide. Thus, the presence of thesodium sulfoxylate formaldehyde within the critical concentration rangein the polymerization reactor reduced the amount of hydrogen cyanideproduced to less than one-sixth of the amount produced in the controlrun of Example 1.

EXAMPLE 3 Example 2 was repeated except that higher amounts of sodiumsulfoxylate formaldehyde were used. In one run, where 0.6 part of sodiumsulfoxylate formaldehyde per 100 parts of monomer was added, theconversion of monomer to polymer decreased to about 60%. In another run,where 0.8 part of sodium sulfoxylate formaldehyde per 100 parts ofmonomer was added, there was little or no conversion of monomer topolymer.

EXAMPLE 4 After equilibrium conditions had been attained, a sample ofthe polymerization reactor overflow slurry of Example 1 (which containedwater, acrylonitrile polymer, unreacted monomers, catalyst residues,etc.) was treated immediately after leaving the polymerization reactorwith sufficient 5% aqueous sodium hydroxide solution to raise the pH to6.0-6.5. This is representative of a prior art shortstopping technique.The sample was then allowed to stand for 30 minutes after which oratedto dryness, and the residue analyzed by infrared technology for nitrile.From this analysis, the amount of acrylonitrile lost as water-solubleacrylonitrile-bisulfite addition product was calculated as about 8.0% onweight of polymer produced.

EXAMPLE 5 Example 4 was repeated except that only sufficient 5% aqueoussodium hydroxide solution to raise the pH to 4.5-5.0 was added. Thisslightly reduced the acrylonitrile loss as water-solubleacrylonitrile-bisulfite addition product to about 6.0% on weight ofpolymer produced.

EXAMPLE 6 Example 4 was repeated except that instead of using sodiumhydroxide, sufficient 5% aqueous sodium sulfoxylate formaldehydesolution was added to amount to an addition of 1% sodium sulfoxylateformaldehyde on weight of unreacted monomers in the slurry. The pH wasunaffected and remained at about 2.5. After allowing the sample to standfor minutes, filtering, evaporating to dryness, and testing the residueas in Example 4, the amount of acrylonitrile lost as water-solubleacrylonitrile-bisulfite addition product was calculated as about 06-08%on weight of polymer produced. Thus, the use of sodium sulfoxylateformaldehyde as the shortstopping agent reduced the of acrylonitrilemonomer as acrylonitrile-bisulfite addition product to less thanone-tenth of the amount lost in the representative prior art run ofExample 4.

Similar results are obtainable when the slurry of Example 2 isshortstopped with sodium sulfoxylate formaldehyde instead of the slurryof Example 1.

I claim:

I. In process for polymerizing a major proportion of acrylonitrile,alone or with one or more ethylenically unsaturated monomerscopolymerizable therewith, in aqueous system using a water-soluble redoxcatalyst system comprising an oxidizing catalyst and a reducingbisulfite activator, the improvement comprising introducing into saidaqueous system 0.1 to 0.5 parts of sodium sulfoxylate formaldehyde perparts of monomer and conducting the polymerization process in thepresence thereof.

2. A process as defined in claim 1 wherein said oxidizing catalyst is analkali metal chlorate.

3. In a continuous process for polymerizing a major proportion ofacrylonitrile, alone or with one or more ethylenically unsaturatedmonomers copolymerizable therewith, wherein water, monomer, and redoxcatalyst system comprising an oxidizing catalyst and a reducingbisulfite activator are continuously introduced into a polymerizationreaction zone and wherein polymerization slurry is continuouslywithdrawn therefrom. the improvement comprising also continuouslyintroducing into said polymerization reaction zone O.l to 0.5 parts ofsodium sulfoxylate formaldehyde per 100 parts monomer introduced.

4. A process as defined in claim 3 wherein said sodium sulfoxylateformaldehyde is introduced as an aqueous solution.

5. A process as defined in claim 3 wherein sufficent sodium sulfoxylateformaldehyde is added to said polymerization slurry after withdrawalfrom said polymerization reaction zone to stop further polymerization.

6. A process as defined in claim 5 wherein about 1% on weight onunreacted monomer of said sodium sulfoxylate formaldehyde is added tosaid slurry.

1. IN PROCESS FOR POLYMERIZING A MAJOR PROPORTION OF ACRYLONITRILE,ALONE OR WITH ONE OR MORE ETHYLENICALLY UNSATURATED MONOMERSCOPOLYMERIZABLE THEREWITH, IN AQUEOUS SYSTEM UNSING A WATER-SOLUBLEREDOX CATALYST SYSTEM COMPRISING AN OXIDIZING CATALYST AND A REDUCINGBISULFITE ACTIVATOR, THE IMPROVEMENT COMPRISING INTRODUCING INTO SAIDAQUEOUS SYSTEM 0.1 TO 0.5 PARTS OF SODIUM SULFOXYLATE FORMALDEHYDE PER100 PARTS OF MONOMER AND CONDUCTING THE POLYMERIZATION PROCESS IN THEPRESENCE THEREOF,
 2. A process as defined in claim 1 wherein saidoxidizing catalyst is an alkali metal chlorate.
 3. In a continuousprocess for polymerizing a major proportion of acrylonitrile, alone orwith one or more ethylenically unsaturated monomers copolymerizabletherewith, wherein water, monomer, and redox catalyst system comprisingan oxidizing catalyst and a reducing bisulfite activator arecontinuously introduced into a polymerization reaction zone and whereinpolymerization slurry is continuously withdrawn therefrom, theimprovement comprising also continuously introducing into saidpolymerization reaction zone 0.1 to 0.5 parts of sodium sulfoxylateformaldehyde per 100 parts monomer introduced.
 4. A process as definedin claim 3 wherein said sodium sulfoxylate formaldehyde is introduced asan aqueous solution.
 5. A process as defined in claim 3 whereinsufficent sodium sulfoxylate formaldehyde is added to saidpolymerization slurry after withdrawal from said polymerization reactionzone to stop further polymerization.
 6. A process as defined in claim 5wherein about 1% on weight on unreacted monomer of said sodiumsulfoxylate formaldehyde is added to said slurry.